Comprehensible Input is a language learning theory proposed by Stephen Krashen. It suggests that learners acquire a new language more effectively when they are exposed to language that is slightly above their current proficiency level, often referred to as "i+1." This means that the input should be understandable but still challenging enough to promote learning. The idea is that when learners receive input that they can mostly understand, they are more likely to absorb new vocabulary and grammar naturally. This approach emphasizes the importance of meaningful communication and context, making language acquisition a more engaging and effective process.